1. Field of the Invention
The present invention relates to a projection exposure apparatus and, more particularly, to focus adjustment with respect to variations in the focal position of a projection optical system having a refraction system.
2. Related Background Art
An integral scanning exposure apparatus using a one-to-one projection optical system including a refraction system and a plane mirror is disclosed in Japanese Patent Publication No. 49-35453. In the projection exposure apparatus disclosed in Japanese Patent Laid-Open No. 7-57986, a projection optical system is constituted by a plurality of one-to-one erect projection optical units each consisting of two partial optical systems each including a refraction system and a concave reflecting mirror. A mask and a plate are moved relatively to this projection optical system to perform full-wafer scanning exposure.
In the above conventional technique, a lens as a part of each refraction system in the projection optical system absorbs exposure light during an exposure process. As a result, the temperature of the lens rises, and the focal position of the projection optical system varies. When a projection optical system constituted by a plurality of projection optical units is to be used, in particular, since the pattern densities (average transmittances) of mask pattern regions corresponding to the respective projection optical units differ from each other, the densities of energy passing through the respective projection optical units differ from each other. For this reason, the temperatures of the refraction systems of the respective projection optical units rise differently, and the focal positions of the respective projection optical units differ from each other.